@StoneyB But what differentiates it from the subject? The fact that the subject comes first, or that it is a "direct object" of the verb (eats)?
– leeand00Jan 8 '18 at 17:59

1

It is the object primarily because of word position. Also, "Mr. Potato Head" is the only singular noun, so only it may be the subject of "eats" (which requires a singular subject, but can take a singular or plural object).
– DavoJan 8 '18 at 18:14

2

@leeand00 The subject performs the action of the verb, the object is affected by the action. In "Jack drives the car" Jack is doing the driving and the car is being driven. In "Mr Potato Head eats monkeys" Mr Potato Head is doing the eating and the monkeys are being eaten.
– BoldBenJan 8 '18 at 18:16

The phrase, "Mr. Potato Head eats monkeys." contains subject, verb, object, in that order. This is typical of an English sentence in the active voice.

As also noted in comments, Mr. Potato Head is singular, which fits with the verb singuar conjugation of eats, whereas monkeys is plural. And in English you cannot write e.g. Mr. Potato Head eat monkeys., where monkeys is the subject and Mr. Potato Head is the object, nor can you write Monkeys eats Mr. Potato Head, with the subject after the verb (compared with German where you may normally write Mr. Potato Head isst Affen but could also write Affen isst Mr. Potato Head since the grammar rules do allow that).